The present invention generally relates to memory arrays of storage cells, and more particularly to a magnetic random access memory (commonly termed xe2x80x9cMRAMxe2x80x9d).
MRAM technology utilizes storage cells, which may hereinafter sometimes be called MTJ (Magnetic Tunnel Junction) junctions or simply xe2x80x9cjunctionsxe2x80x9d which each have at least two magnetic regions or layers with an electrically insulating barrier layer between them. The storage mechanism relies on the relative orientation of the magnetization of the two layers, and on the ability to discern this orientation by electrical means through electrodes attached to these layers. For background, reference made to U.S. Pat. Nos. 5,650,958 and 5,640,343 issued to William Joseph Gallagher et al on Jul. 22, 1997 and Jun. 17, 1997, respectively.
MRAM memory arrays include an array of magnetic memory cells or data storage cells (e.g., cell 50) positioned at the intersections of wordlines 1, 2, 3 and bitlines 4, 5, 6, as shown in FIG. 1.
In a preferred form, each cell includes a magnetically changeable (reversible) or xe2x80x9cfreexe2x80x9d region, and a proximate magnetically reference or xe2x80x9cfixedxe2x80x9d region, arranged into a magnetic tunnel junction (xe2x80x9cMTJxe2x80x9d) device (e.g., the term xe2x80x9creference regionxe2x80x9d is used broadly herein to denote any type of region which, in cooperation with the free or changeable region, results in a detectable state of the device as a whole).
Generally, the principle underlying storage of data in such cells is the ability to change, and even reverse, the relative orientation of the magnetization of the free and reference regions by changing the direction of magnetization along the easy axis (xe2x80x9cEAxe2x80x9d) of the free region, and the ability to thereafter read this relative orientation difference.
More particularly, an MRAM cell is written by reversing the free region magnetization using applied bi-directional electrical and resultant magnetic stimuli via its respective bitline and wordline.
The MRAM cell is later read by measuring the resultant tunneling resistance between the bitline and wordline, which assumes one of two values depending on the relative orientation of the magnetization of the free region with respect to the reference region. If the free region is modeled as a simple elemental magnet having a direction of magnetization which is free to rotate but with a strong preference for aligning in either direction along its easy axis (+EA or xe2x88x92EA), and if the reference region is a similar elemental magnet but having a direction of magnetization fixed in the +EA direction, then two states (and therefore the two possible tunneling resistance values) are defined for the cell: aligned (+EA/+EA) and anti-aligned (xe2x88x92EA/+EA).
An ideal hysteresis loop characterizing the tunnel junction resistance with respect to the applied EA field is shown in FIG. 2. The resistance of the tunnel junction can assume one of two distinct values with no applied stimulus in region 20 (e.g., there is a lack of sensitivity of resistance to applied field below the easy axis flipping field strength xc2x1Hc in region 20).
For example, if the applied easy axis field exceeds xc2x1Hc, then the cell is coerced into its respective high resistance (anti-aligned magnetization of the free region with respect to the reference region) or low resistance (aligned magnetization of the free region with respect to the reference region) state.
Thus, in operation as a memory device, the MRAM device can be read by measuring the tunneling resistance, thereby to infer the magnetization state of the storage layer with respect to the fixed layer.
Thus, FIG. 2 illustrates an ideal hysteresis loop for the MRAM junction of the measured resistance versus applied easy axis filed for an ideal magnetic tunnel junction device. Such a resulting hysteresis loop, shown schematically in FIG. 2, is desirable, since the resistance has one of two distinct possibilities, and there is a lack of sensitivity of measured resistance to applied field below the flipping field strength HC. However, in a practical device, such ideal behavior oftentimes fails to exist, thereby raising many problems.
FIGS. 3(a) and 3(b) illustrate a geometry typically proposed for addressing the MRAM cell 50. The bias current indicated by the arrow through the cell 50 flows from bit line 5 to word line 2, as shown in FIG. 3(a), while the magnetic field-generating current flows in either bit line 5 or word line 2 as indicated by the arrows, but not between the two, as shown in FIG. 3(b). Obviously, there may be other considerations, but these are not considered germane to the present invention.
Although the ability to cleanly write the bit (corresponding to the reversal of the free or storage layer in the MRAM geometry discussed above) is important, there is another requirement for the successful operation of the MRAM array.
Specifically, it is necessary to choose, for writing data, only one of the many cells in the array, but without disturbing any of the other cells during this write process.
FIG. 4 illustrates an astroid shape (a so-called xe2x80x9castroid plotxe2x80x9d or Stoner-Wolfarth astroid) and selective switching of cells in an array.
The solid line 40 traces the boundaries of stability for a single idealized particle for magnetization pointing either left or right as a function of applied magnetic field. The axes of the plot correspond to the easy and hard axis field (e.g., parallel or perpendicular to the direction preferred by the crystalline anisotropy).
Inside the astroid boundary, there are two stable states, and depending on magnetic history either can be achieved. However, outside the astroid, there is only one state of magnetization which is parallel to the applied magnetic field. Because of the shape of the astroid, the magnetic field may be used to isolate a particular data storage cell for writing.
As shown by the dotted lines forming a box 41 in FIG. 4, easy and hard axis fields (generated by currents through the bit and word lines intersecting at a selected cell) of amplitude HW force the junction into the right-pointing state. Neighboring junctions (e.g., those either on the same word line or same bit-line) having either insufficient (i.e. Within their asteroid boundaries) easy or hard axis fields are not expected to change state. Thus, as is known, a specific selected device can be written by applying current to both the word and bit lines.
Unfortunately, the above procedure for writing requires very tight manufacturing tolerances.
Specifically, in practice, the solid line 40 of the asteroid expands into a band when considering the range of stability for a population of junctions. If this band becomes too large, then there is no combination of easy and hard axis fields which will definitely (and surely) switch any desired junction, while simultaneously definitely not switching any other junction by mistake. Thus, reliability becomes a problem.
Further, the magnetic field at a junction is due not only to the fields from the bit and word lines, but also to the magnetic state of the junctions around it. This effect, when considered statistically, further reduces the window of operation.
In view of the foregoing and other problems of the conventional methods and structures, it is an object of the present invention to provide a method of increasing accuracy in the writing process and that reduces the type of errors described above.
Broadly, the present invention therefore provides a memory array of storage cells comprising:
a) an array of electrically conducting bit lines and electrically conducting word lines which form a plurality of intersections therebetween,
b) a storage cell (e.g. an MTJ) disposed at each of the aforesaid intersections, the storage cell comprising at least one changeable (preferably reversible) magnetic region characterized by a magnetization state which can be changed (preferably reversed) by applying thereto a selected external magnetic field, the aforesaid changeable (e.g. reversible) magnetic region comprising a material whose magnetization state is more easily changed (e.g. reversed) upon a change in the temperature thereof, and
c) a heat generator for heating the aforesaid changeable (e.g. reversible) magnetic region of only a selected one of the aforesaid array of storage cells at any moment.
Preferably, each cell further comprises at least one fixed magnetic region characterized by a magnetization state which cannot be changed or reversed by applying the aforesaid selected external magnetic field to the fixed magnetic region.
According to another preferred embodiment, in addition to the aforesaid changeable magnetic region, each cell further comprises a second xe2x80x9cinterrogationxe2x80x9d magnetic region whose magnetization can be changed (with or without the assistance of thermal changes). During a read operation, the interrogation layer is set in one or more known directions to sense the orientation of the changeable layer.
According to preferred embodiments of the invention, selective heating is applied at the selected storage cell. For this purpose, the aforesaid heat generator comprises a voltage source operable to heat the aforesaid reversible magnetic region (e.g. layer) by passing electric tunnelling current therethrough to cause Joule heating in that selected cell. Preferably, this tunnelling current is a writing current between a bit line and a word line for writing data into the aforesaid selected storage cell.
Preferably, the aforesaid writing current is passed through the aforesaid reversible magnetic layer for a time period (e.g. as a pulse) which is chosen to be sufficiently short to prevent reversal of the magnetization state of other cells of said memory array by thermal leakage thereto.
According to a preferred embodiment of the invention, the aforesaid reversible magnetic region comprises a ferrimagnetic material, such as Gd23Fe77, Gd24Fe76, Tb19Fe81, Tb21Fe79, Dy17Fe83, and Dy21Fe79. Magnetic alloys of other metals, such as Co with Sm, may also be used.
As will be understood, it may be desirable to maintain the aforesaid reversible magnetic region at the compensation temperature of the the aforesaid ferrimagnetic or other material to maintain stored data in the memory array.